Re: [mm/gup] 47e29d32af: phoronix-test-suite.npb.FT.A.total_mop_s -45.0% regression

From: Jan Kara <jack@suse.cz>
To: John Hubbard <jhubbard@nvidia.com>
Cc: "kernel test robot" <oliver.sang@intel.com>,
	"Linus Torvalds" <torvalds@linux-foundation.org>,
	"Jan Kara" <jack@suse.cz>,
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	"Jérôme Glisse" <jglisse@redhat.com>,
	"Matthew Wilcox (Oracle)" <willy@infradead.org>,
	"Al Viro" <viro@zeniv.linux.org.uk>,
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Subject: Re: [mm/gup] 47e29d32af: phoronix-test-suite.npb.FT.A.total_mop_s -45.0% regression
Date: Wed, 18 Nov 2020 14:49:52 +0100	[thread overview]
Message-ID: <20201118134952.GE1981@quack2.suse.cz> (raw)
In-Reply-To: <24d9d093-5b7a-9aee-8d61-59c0007a9269@nvidia.com>

On Mon 16-11-20 19:35:31, John Hubbard wrote:
> 
> On 11/16/20 6:48 PM, kernel test robot wrote:
> > 
> > Greeting,
> > 
> > FYI, we noticed a -45.0% regression of phoronix-test-suite.npb.FT.A.total_mop_s due to commit:
> > 
> 
> That's a huge slowdown...
> 
> > 
> > commit: 47e29d32afba11b13efb51f03154a8cf22fb4360 ("mm/gup: page->hpage_pinned_refcount: exact pin counts for huge pages")
> > https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
> 
> ...but that commit happened in April, 2020. Surely if this were a serious
> issue we would have some other indication...is this worth following up
> on?? I'm inclined to ignore it, honestly.

Why this was detected so late is a fair question although it doesn't quite
invalidate the report... The NPB benchmark appears to be a supercomputing
benchmark so concievably it could be heavily using THPs. The question is
why it would be a heavy user of pinning as well but even that is imaginable
considering that MPI is in use etc.

So maybe it is worth trying to reproduce this because heavy THP + pinning
users might be indeed rare and only those would show regressions in THP
pinning performance...

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR